Thermal bubble-forming inkjet printheads function by generating a certain amount of heat in the printing fluid contained in a nozzle chamber. This heat causes a bubble to form which eventually collapses as the fluid is forced through a nozzle. The collapse of the bubble then causes more fluid to enter the nozzle chamber for the same process to begin again.
Due to the ability of water to form rapidly a bubble at elevated temperature, printers equipped with such thermal inkjet printheads typically use water-based ink formulations.
However, although it is possible to print directly onto specially treated non-porous and semi-porous substrates, water-based inkjet inks are essentially limited to applications that use porous media, such as office papers and textiles. Accordingly, most printers that use thermal inkjet printheads are usually for the SOHO (small office home office) market.
In contrast, piezo printheads offer a wide degree of ink formulation flexibility, since droplet ejection occurs through a piezoelectric force rather than a bubble forming mechanism. Because of this, many different ink chemistries can be used in piezo printheads, such as UV curable, solvent-based, hot-melt and oil-based, as well as water-based. The output from the piezo printers is equally diverse, addressing a host of graphics market segments including packaging, wide-format displays, CD and glass decoration. Taking wide-format printing as an example, there are a large number of solvent-based printers on the market all of which use piezo printheads. These print onto a range of media, including low cost non-porous and semi-porous substrates such as uncoated vinyls and polyesters.
Solvent-based inkjet inks comprising pigments, as used in piezo printheads, contain high concentrations of polymer to help form stable dispersions of appropriate particle size.
For example, commercially-available pigments for solvent-based inkjet ink applications, such as those from the Microlith range (Ciba), consist of pigments pre-dispersed in a vinyl chloride/vinyl acetate copolymer binder. In order to formulate a typical solvent-based inkjet ink, the commercially-available pigment dispersions are “let-down” in solvent and additional polymer. Incorporation of the additional polymer improves film properties and helps maintain particle stability. However, in thermal printheads, high concentrations of polymer are undesirable because they tend to kogate on the heater element, reduce ink latency and/or decrease drop velocity.
The Applicant has developed a range of bend-actuated printheads, which do not rely on bubble formation for ink ejection and, like piezo printheads, can be used with either aqueous or non-aqueous inks. Some of the Applicant's bend-actuated inkjet printheads are described in, for example, U.S. Pat. Nos. 7,416,280; 6,902,255; US 2008/0129793; and US 2008/0225082, the contents of which are herein incorporated by reference.
The present Applicant has also developed a range of thermal bubble-forming printheads, as described in, for example, U.S. Pat. Nos. 6,755,509; 7,246,886; and 7,401,910, the contents of which are incorporated herein by reference.
The Applicant's thermal bubble-forming inkjet printers currently offer high-resolution and high-speed printing (e.g. 60 pages per minute at 1600 dpi) of aqueous-based inks for SOHO markets. These aqueous-based inks are suitable for printing onto porous and semi-porous media. However, it would be desirable to provide thermal bubble-forming inkjet printheads, which can print ink onto a wider range of substrates, including non-porous media. This would allow thermal inkjet printheads to compete fully with piezo printers, offering high-resolution and high-speed inkjet printing for a wide range of print media.